Many electric linear-motion actuators for converting the rotary motion of an electric motor to a linear motion, thereby linearly driving a member to be driven, include a ball-screw mechanism or a ball-ramp mechanism as their motion converter mechanism. Also, in order to obtain large linear driving force with a small-capacity electric motor, many of such actuators include a speed reducing mechanism such as a planetary gear speed reducing mechanism (see e.g. JP patent publication 6-327190A (FIGS. 1 and 5)).
On the other hand, many of the existing vehicle brake assemblies are hydraulic ones. But with the recent introduction of sophisticated brake control systems such as anti-lock brake systems (ABS), electric brake assemblies are gathering attention because they can perform such sophisticated control without the need for complicated hydraulic circuits and they can be designed compactly. Such electric brake assemblies include an electric motor which is actuated in response to e.g. signals indicating that the brake pedal is depressed, and an electric linear-motion actuator as described above which is mounted in a caliper body for pressing a brake member against the member to be braked when the motor is actuated (see e.g. JP patent publication 2003-343620A (FIG. 1)).
Ordinarily, electric brake assemblies are mounted on a vehicle each under one of the springs of the vehicle, and thus, it is desired that such brake assemblies operate stably under the influence of vibrations transmitted from the ground, and can be designed compactly.